[Some say, Brahman is everything. Does this mean, even non living objects like rocks, mountains, rivers, chairs, cars, are they all Brahman?]
This claim cannot be debated. Either one has experienced it and knows or one doesn't know.
There is this Gita verse that says that all is Brahman.
To one of the above description, the ladle with which the offering is
made and the oblations are Brahman; and the sacrificial rites (which
is Brahman) is performed by the sacrifice who is Brahman, in the fire
which too is Brahman. He who is thus absorbed in work as Brahman,
attains to Brahman alone.
Gita 4.24
Then there is a record of Swami Vivekananda (Narendra) directly experiencing that state.
During his second visit, about a month later, suddenly, at the touch
of the Master, Narendra felt overwhelmed and saw the walls of the room
and everything around him whirling and vanishing. "What are you doing
to me?" he cried in terror. "I have my father and mother at home." He
saw his own ego and the whole universe almost swallowed in a nameless
void. With a laugh the Master easily restored him. Narendra thought he
might have been hypnotized, but he could not understand how a
monomaniac could cast a spell over the mind of a strong person like
himself. He returned home more confused than ever, resolved to be
henceforth on his guard before this strange man.
But during his third visit Narendra fared no better. This time, at the
Master's touch, he lost consciousness entirely. While he was still in
that state, Sri Ramakrishna questioned him concerning his spiritual
antecedents and whereabouts, his mission in this world, and the
duration of his mortal life. The answers confirmed what the Master
himself had known and inferred. Among other things, he came to know
that Narendra was a sage who had already attained perfection, and that
the day he learnt his real nature he would give up his body in yoga,
by an act of will.
A few more meetings completely removed from Narendra's mind the last
traces of the notion that Sri Ramakrishna might be a monomaniac or
wily hypnotist. His integrity, purity, renunciation, and unselfishness
were beyond question. But Narendra could not accept a man, an
imperfect mortal, as his guru. As a member of the Brahmo Samaj, he
could not believe that a human intermediary was necessary between man
and God. Moreover, he openly laughed at Sri Ramakrishna's visions as
hallucinations. Yet in the secret chamber of his heart he bore a great
love for the Master.
Sri Ramakrishna was grateful to the Divine Mother for sending him one
who doubted his own realizations. Often he asked Narendra to test him
as the money-changers test their coins. He laughed at Narendra's
biting criticism of his spiritual experiences and samadhi. When at
times Narendra's sharp words distressed him, the Divine Mother Herself
would console him, saying: "Why do you listen to him? In a few days he
will believe your every word." He could hardly bear Narendra's
absences. Often he would weep bitterly for the sight of him. Sometimes
Narendra would find the Master's love embarrassing; and one day he
sharply scolded him, warning him that such infatuation would soon draw
him down to the level of its object. The Master was distressed and
prayed to the Divine Mother. Then he said to Narendra: "You rogue, I
won't listen to you any more. Mother says that I love you because I
see God in you, and the day I no longer see God in you I shall not be
able to bear even the sight of you."
The Master wanted to train Narendra in the teachings of the
non-dualistic Vedanta philosophy. But Narendra, because of his Brahmo
upbringing, considered it wholly blasphemous to look on man as one
with his Creator. One day at the temple garden he laughingly said to a
friend: "How silly! This jug is God! This cup is God! Whatever we see
is God! And we too are God! Nothing could be more absurd." Sri
Ramakrishna came out of his room and gently touched him. Spellbound,
he immediately perceived that everything in the world was indeed God.
A new universe opened around him. Returning home in a dazed state, he
found there too that the food, the plate, the eater himself, the
people around him, were all God. When he walked in the street, he saw
that the cabs, the horses, the streams of people, the buildings, were
all Brahman. He could hardly go about his day's business. His parents
became anxious about him and thought him ill. And when the intensity
of the experience abated a little, he saw the world as a dream.
Walking in the public square, he would strike his head against the
iron railings to know whether they were real. It took him a number of
days to recover his normal self. He had a foretaste of the great
experiences yet to come and realized that the words of the Vedanta
were true.
The Gospel of Sri Ramakrishna translated by Swami Nikhilananda,, Introduction, Narendra
[Some say, Brahman is the atman inside us (living beings) and those who realize their own soul to be the eternal aspect, attain moksha. Does this mean, advaitists are atheists, since they don't consider a higher creator entity and consider their own soul to be the only object of realization ? ... If there's no God as per advaita but only embodied atmas as Brahman, then does this mean, we atmas as a whole created galaxies, solar systems, planets etc. before entering into gross flesh bodies?]
No, it does not mean advaitists are atheists. It means they are non-theists. What does this mean?
Do you know what I mean? Think of Brahman, Existence-Knowledge-Bliss
Absolute, as a shoreless ocean. Through the cooling influence as it
were, of the bhakta's love, the water has frozen at places into blocks
of ice. In other words, God now and then assumes various forms for His
lovers and reveals Himself to them as a Person. But with the rising of
the sun of knowledge, the blocks of ice melt. Then one doesn't feel
any more that God is a Person, nor does one see God's forms. What He
is cannot be described. Who will describe Him? He who would do so
disappears. He cannot find his 'I' anymore.
The Gospel of Sri Ramakrishna, The Master with Brahmo Devotees (I), October 28, 1882
The above passage is best understood if you think of the Earth and its frozen poles but minus its lands. Theism deals with the realm of icebergs floating in the Northern and Southern oceans. Only in the frozen areas can one talk of the universe and its living beings and different forms of God. Different theistic systems claim different icebergs to be the Supreme Being. Advaitists are those who want to be in the iceberg free open ocean. No concept of form exists in this realm. There is only formless water everywhere. There is no individual ego either. Thus there is no way one can have a creator God in this realm.
[And finally, there are advaitists, who say Brahman is pure consciousness. Now i'm going to ask who's consciousness? Individual consciousness or universal consciousness? And what is meant by universal consciousness.]
There is no other person when one reaches the state of Nirguna Brahman. In that state Brahman is all.
If one analyzes oneself, one doesn't find any such thing as 'I'. Take
an onion, for instance. First of all peel off the red outer skin; then
you find thick white skins. Peel these off one after the other, and
you won't find anything inside.
In that state a man no longer finds the existence of his ego. And who
is there left to seek it? Who can describe how he feels in that state
- in his own Pure Consciousness - about the real nature of Brahman?
There is a sign of Perfect Knowledge. Man becomes silent when It is
attained. Then the 'I', which may be likened to the salt doll, melts
in the ocean of Existence-Knowledge-Bliss Absolute and becomes one
with It. Not the slightest distinction is left.
The Gospel of Sri Ramakrishna, The Master with Brahmo Devotees (I), October 28, 1882
The statement that Brahman is pure consciousness is only an analogy. Since we see conscious beings and Brahman is the source of all consciousness it must be pure consciousness. Actually nothing can be said about Brahman.
"What Brahman is cannot be described. All things in the world - the
Vedas, the Puranas, the Tantras, the six systems of philosophy - have
been defiled, like food that has been touched by the tongue. Only one
thing has not been defiled in this way, and that is Brahman. No one
has ever been able to say what Brahman is.Brahman is beyond word and
thought. It is said in the Vedas that Brahman is of the nature of
Bliss. It is Satchidananda. In Samadhi one attains the knowledge of
Brahman - one realizes Brahman. In that state reasoning stops
altogether, and man becomes mute. He has no power to describe the
nature of Brahman."
The Gospel of Sri Ramakrishna, Visit to Vidyasagar, August 5, 1882
The Upanishad also declares Brahman to be Consciousness alone. The
Upanishad also says that Brahman is pure consciousness, devoid of
other aspects contrary to this, and without any distinguishing
features, as in, “As a lump of salt is without interior or exterior,
entire, and purely saline in taste, even so is the Self without
interior or exterior, entire, and pure Intelligence alone”
(Brhadaranyaka Upanishad IV.v.13), which means that the Self has no
internal or external aspect apart from pure consciousness, Its nature
being mere impartite consciousness without any interstices. Just as a
lump of salt has the saline taste alone both inside and outside, and
no other taste, so also is this Self.
Brahma Sutra Bhasya of Sri Sankaracharya III.ii.16
Moreover, the Vedas reveal this; likewise this is mentioned in the
Smritis also.Moreover, the Vedas reveal through a negation of other
aspects that Brahman has no distinguishing feature, as for instance
in, “Now therefore the description (of Brahman): ‘Not this, not this’”
(Brhadaranyaka Upanishad II.iii.6), “That (Brahman) is surely
different from the known; and, again, It is above the unknown” (Kena
Upanishad I.4), “That Bliss of Brahman, failing to reach which, words
turn back along with the mind” (Taittiriya Upanishad II.ix.1), and so
on. And it is also known from the Vedic texts that Badhva being asked
by Baskali, replied merely by not uttering a word, as stated in, “He
(Baskali) said, ‘Teach me Brahman, sir.’ He (Badhva) became silent.
When the question was repeated a second and a third time he said, ‘I
have already spoken, but you cannot comprehend. That Self is
Quiescence’ “. Similarly in the Smritis, the instruction is given
through a negation of other things, as in, “I shall tell you of that
which is to be known and by knowing which one attains immortality. The
supreme Brahman is without any beginning. It can neither be called
gross (visible) nor fine (invisible)” (Gita XIII.12), and so on.
Similarly the Smriti mentions how Narayana in His cosmic form said to
Narada, “O Narada, that you see me as possessed of all the (five
divine) qualities of all elements, is only because of My Maya, called
up by Myself. For else you should not understand Me thus.”
Brahma Sutra Bhasya of Sri Sankaracharya III.ii.17
Brahman defeats all attempt to describe It and hence the silence.